The correct answer is option 'C': Unicellular zygote.

1. Unicellular Zygote:
Sexually reproducing multicellular animals begin their development from a unicellular zygote. The zygote is formed when a sperm cell fertilizes an egg cell during the process of sexual reproduction. The zygote is a single cell that contains the genetic material from both parents.

2. Fertilization:
Fertilization occurs when a sperm cell penetrates the outer protective layers of the egg cell and fuses with the egg's nucleus. This fusion of genetic material from both parents results in the formation of a unicellular zygote.

3. Cleavage:
After fertilization, the zygote undergoes a process called cleavage. Cleavage is a series of rapid cell divisions without cell growth. These divisions result in the formation of a solid ball of cells called a morula.

4. Morula:
The morula is a compact mass of cells that continues to divide and undergo further changes. It is formed by the repeated division of the zygote and consists of cells known as blastomeres.

5. Blastula:
As the morula continues to divide, it undergoes a process called blastulation, which leads to the formation of a hollow ball of cells called a blastula. The blastula consists of an outer layer of cells called the trophoblast and an inner cell mass.

6. Gastrulation:
Gastrulation is the next major event in development. During gastrulation, the blastula undergoes complex rearrangements of cells, leading to the formation of three distinct germ layers: the ectoderm, mesoderm, and endoderm.

7. Organogenesis:
Following gastrulation, the three germ layers differentiate and give rise to specific tissues and organs through a process called organogenesis. The ectoderm gives rise to the nervous system, skin, and other structures. The mesoderm forms muscles, connective tissues, and other organs. The endoderm gives rise to the digestive tract, respiratory system, and other internal organs.

8. Developmental stages:
From this point onwards, the embryo goes through various stages of development, including the formation of specific organs and systems, until it eventually develops into a fully formed multicellular animal.

In summary, sexually reproducing multicellular animals start their development from a unicellular zygote formed through the fertilization of an egg cell by a sperm cell. The zygote undergoes cleavage, forming a morula, which then develops into a blastula through blastulation. Gastrulation occurs next, leading to the formation of three germ layers, which further differentiate and give rise to specific tissues and organs through organogenesis.